Page:The Osteology of the Reptiles.pdf/127

Rh, two, has remained persistent in most reptiles and even most mammals to the present time. A third vertebra, from the caudal series, was early united in many Theromorpha and the latest Cotylosauria. Still another, and possibly two, were joined in the Dinocephalia and Anomodontia. The Plesiosauria, purely aquatic animals with propelling legs, have three or four sacrals. From one to three additional vertebrae have been fused with the sacrum in front in the Pterosauria (Fig. 118 ), and some Dinosauria, but they are not true sacral vertebrae.

Not only may the sacral vertebrae be closely fused, but their arches and spines may become almost indistinguishably united. Usually, however, the zygapophyses remain visible and are sometimes functional. In Iguana, even the zygosphene and zygantrum are present between the two sacrals. The sacrum is lost, not only in the snakes and legless lizards, but also in the mosasaurs and late ichthyosaurs, where hind legs have lost locomotive functions.

The tail of the earliest known reptile, from the Coal Measures of Ohio (Fig. 84), was long and slender. The Cotylosauria had, for the most part, only a moderately long tail, with not more than sixty vertebrae. The length of the tail, however, depends so much upon habits that it may be extremely variable even in members of the same order. Stumpy-tailed lizards (Trachysaurus), for instance, have practically no tail, while other skinks have a very long and slender one. Invariably it is long in tail-propelling, swimming reptiles; such reptiles move sinuously through the water, preventing much use of the legs as propelling organs. Those with propelling legs, on the other hand, have a broader and flatter body and short tail, of use only as a steering organ. However, sauropod dinosaurs, though supposed to be exclusively water animals, have a very long and slender tail, more or less whiplash-like at the end. As a rule, swift-moving, crawling reptiles have a long and slender tail, while short-tailed reptiles are invariably slow in their movements upon land.

The spines of the caudal vertebrae in land reptiles are seldom long; certain chameleon lizards and the basilisc lizard are exceptions; the